The present invention relates in general to a supporting electrode plate adapted to be soldered to a semiconductor substrate. In particular, the invention concerns a method of fabricating a supporting electrode plate for a semiconductor device, which electrode is made of a composite material composed of copper (Cu) or a copper alloy (Cu-alloy) and carbon (C) fibers.
The material for the electrode plate adapted to be soldered to a semiconductor device or substrate thereof for supporting it is usually required to exhibit following properties.
(1) The thermal expansion coefficient of the electrode plate material has to be substantially equal to that of the semiconductor substrate to which the electrode is soldered.
(2) The electrode plate material has to exhibit excellent thermal and electrical conductivities.
The supporting electrode plate is bonded to an associated semiconductor substrate usually by soldering. Accordingly, when the thermal expansion coefficients of the materials for the electrode and the semiconductor device differ remarkably from each other, the material which is more fragile or of lower mechanical strength tends to undergo warp or destruction due to difference in the thermal expansion when the bonded portion is cooled after the soldering or when heat is generated during the operation of the semiconductor device. In the case where silicon (Si) is employed for the semiconductor material, the destruction or warp will take place in the thin layer of Si as a rule. As is well known, silicon or Si is a material exhibiting an extremely small thermal expansion, the coefficient of which is about 3.5.times.10.sup.-6 /.degree.C. in the temperature range of a room temperature to 250.degree. C. Thus, it is required that the material intended for the supporting electrode plate bonded to a semiconductor silicon substrate should exhibit a low thermal expansion, the coefficient of which is correspondingly about 3.5.times.10.sup.-6 /.degree.C.
Additionally, the supporting electrode plate has to serve for dissipating Joule heat generated in the semiconductor body during the operation of the semiconductor device. For this reason, the material for the supporting electrode plate has to be excellent in respect of the thermal conductivity in addition to a high electrical conductivity. Such requirement may be adequately satisfied when pure Cu or an alloy thereof which has thermal and electrical conductivities approximating to those of pure Cu can be used as the material for the supporting electrode plate, whereby disadvantages ascribable to poor dissipation of Joule heat such as increased current leakage, increase in the forward voltage drop or the like performance degradations can be effectively excluded.
Heretofore, tungsten (W) or molybdenum (Mo) has been used for the supporting electrode plate and found to be effective for preventing the destruction and warping or bulging from occurring in the semiconductor substrate of Si. Tungsten (W) has a thermal expansion coefficient of about 4.4.times.10.sup.-6 /.degree.C. in the temperature range of a room temperature to 250.degree. C., while that of molybdenum is about 5.5.times.10.sup.-6 /.degree.C. Thus, both material satisfy the above requirements in respect of the thermal and electrical conductivities to a reasonable degree.
It goes however without saying that further improvement on the thermal and electrical conductivities of the supporting electrode plate is desirable, since then the output of the semiconductor device can be increased with the reliability being correspondingly enhanced. In U.S. patent application Ser. No. 911,078, assigned to the assignee of the present invention a composite material composed of Cu or a Cu-alloy and carbon (C) fibers are proposed as a material for the supporting electrode plate to be used in place of W or Mo. Further, it is taught that carbon (C) fibers contained in the composite material should be preferably arrayed in a spiral or ring-like configuration at least in the surface of the electrode plate at which the electrode plate is bonded to the semiconductor body or substrate. The electrode plate of the composite material which includes Cu and 30% by volume of carbon in form of spiral carbon fibers arrayed substantially coaxially exhibits electrical conductivity of about 50 IACS% which is of course significantly higher than those of W and Mo, i.e. 30 to 35 IACS%. Besides, the coefficient of thermal expansion in the radial direction of the spiral carbon fibers is about 5.times.10.sup.-6 /.degree.C. and more generally 4.5.about.5.3.times.10.sup.-6 /.degree.C. in the temperature range of a room temperature to 250.degree. C. which coefficient is substantially same as those of W and Mo. In this manner, the composite material disclosed in U.S. Patent Application described above allows the thermal and electrical conductivities of the electrode to be enhanced while maintaining the thermal expansion coefficient substantially equal to those of W and Mo.
It should however be mentioned that difficulties have hitherto been encountered in manufacturing the electrode plate of the composite material in which the carbon fibers are spirally and coaxially incorporated. In reality, there is a great demand for establishing a method of fabricating the supporting electrode plate of the composite material in a facilitated manner.